The epitome of the wicked stepmother, from the 1950 version of Cinderella. Photograph: Sportsphoto/Allstar/Disney

You only have to dip into the Disney archive, take a glimpse at Cinderella's evil stepfamily and catch a few frames of Snow White's stepmum, the Queen, to realise that stepmothers traditionally get a poor press. "The idea of the wicked stepmother still affects people's perception hugely," says Dr Lisa Doodson, a psychology lecturer at Thames Valley University. "A lot of women feel they're battling against that image."

Doodson should know. She is not only the mother of an 11-year-old stepson, as well as two biological children, but has spent the last four years interviewing more than 300 stepfamilies to look at how the experience affects women.

"In the past, researchers have looked at the impact of growing up in a stepfamily on children, but there's been very little work on how women who become stepmothers go through the experience," she explains. "There's an assumption that all of their issues and experiences are the same. I wanted to challenge that."

Doodson's interest kicked off when she first met her partner, now husband, and they both took on step-parent roles. "I looked around for advice about things like how to deal with disciplining, and how to define the relationship, but I couldn't find any. Then, as psychologist and stepmother, I grew interested in the different psychological effects of taking on this ill-understood parenting role."

So Doodson began advertising in national newspapers and magazines, school newsletters and on family websites and charities until she had built up a pool of 250 stepmums and 80 biological mothers whose experiences of motherhood could be examined. "People were very keen to share their experiences with me," she says. "They really wanted to use their experiences to help others."

Doodson asked the mums to fill in detailed questionnaires about their wellbeing, their relationship with their partners, how much support they received from family and friends, and their relationship with their stepchildren. She then interviewed many of the participants in greater detail, and identified four types of stepmother and compared their levels of anxiety.

The first category covered mothers with full-time stepkids and full-time care of biological children. The second was those with full-time stepkids but no biological children. Next up was women with no biological children and part-time care of stepkids, and the fourth group was mothers with biological kids as well as part-time responsibility for stepchildren.

Comparing the four groups, Doodson found the highest levels of depression in women who had full-time care of stepkids and biological kids. "It was not hugely surprising," she admits. "The mums have so much to juggle." But there were also high levels of anxiety among stepmums with no children of their own. "They didn't have any experience of looking after kids, and worried about what to do with them, as well as things like whether they should discipline them." This group of stepmums was also confused by the ambiguity of their role, and whether they should act as a mother or a friend to their stepkids. Doodson says the ongoing influence of the biological mother, via endless phone calls or rules, was another source of anxiety.

Overall, the research showed that stepmothers in general have "significantly greater anxiety and depression than biological mothers". Most stepfamilies faced difficulties defining their individual roles. At one focus group, a participant admitted that her stepgrandson didn't know what to call her, so often referred to her as simply "her". "The other stepmums in the focus group were horrified and felt so sad for her," Doodson explains. "We were able to convince her that she could be an extra grandparent even if the child already had the 'full complement' as she believed. In stepfamily situations there really are no rules or social norms to conform to."

Doodson also found that stepmothers have far less social support than biological mothers. "Often grandparents stay in contact with the biological mother out of loyalty – as well as fears about having their contact with their grandchildren cut off – so they don't offer support to the stepmum, who is often left feeling isolated and resentful as a result," she explains. The stepmums without biological children also said they had lost contact with friends, because their new family role left them with little in common with single friends.

Partly to alleviate that problem, Doodson set up a stepfamily parenting course called Happy Steps, for step-parents to receive group support about how to improve their family relationships. She has also used her research to develop courses for family professionals, such as social workers, about the key issues that stepfamilies face.

"When I began studying psychology eight years ago, I didn't have a grand plan of becoming a stepfamily expert. But, partly because of my own experience, and the lack of existing support for stepfamilies, I became more and more passionate about understanding the issues in these often-complex families, and using my research to help them build happier and stronger family units."

• Lisa Doodson's book, How to be a Happy Stepmum, is published by Vermilion on 6 May, £10.99. Find out more about her courses at www.happysteps.co.uk